The present invention relates to a method of purifying brine to be subjected to electrolyzation for producing caustic soda and chlorine, and a system for carrying out the same.
In electrolyzing salt, it is the usual practice is to supply brine, which has been prepared by dissolving salt in water and purified by removing impurities contained in the salt solution, to an electrolyzer. Recently, the electrolytic process for electrolyzing brine has been converted progressively from the membrane system to the ion exchange membrane system due to the increase in demand for electrolytic caustic soda of higher purity; consequently, the purification of brine has become regarded as important.
When brine containing much hardness components is supplied to the electrolytic process of the ion exchange membrane system, the performance of the ion exchange membranes deteriorates and the life of the same is remarkably reduced. Accordingly, the brine needs to be treated, through filtering layers packed with a chelate resin, to achieve high purification before the brine is supplied to an electrolyzer.
FIG. 1 illustrates a conventional brine purifying system. Referring to FIG. 1, dechlorinated dilute brine recovered from an electrolyzer 14 is returned from a dechlorinated dilute brine tank 13 to a saturated brine tank 1. The saturated brine tank 1 is replenished with salt and, if necessary, water to prepare saturated brine.
The saturated brine is supplied to a mixing tank 2, where caustic soda or sodium carbonate is mixed with the saturated brine to produce so-called brine mud. The saturated brine containing the brine mud is supplied to a thickner 3, where most of the brine mud is deposited and the deposited brine mud is discharged from the system from the bottom of the thickner 3.
Since the supernatant saturated brine supplied from the thickner 3 usually contains minute amount of insoluble solid matter, the supernatant saturated brine is stored temporarily in a supernatant saturated brine tank 4, and then the supernatant saturated brine is supplied to a filtering apparatus 6. The filtered and purified brine is stored in a purified brine tank 7, and then the purified brine is supplied to a chelate resin tower 8, where a small amount of the residual impurity ions are removed. Then, hydrochloric acid is added to the purified brine in a pH adjusting tank 9 to adjust the pH of the purified brine. Then, this high purity brine is supplied to the electrolyzer.
However, it has been found that the life of the chelate resin is greatly dependent upon the amount of suspended solid matter remaining in the brine and that a high degree of separation of the solid matter is necessary in addition to the solid matter separation achieved by the thickner. In order to meet such a requirement, purified brine is obtained by separating the solid matter from the saturated brine by means of the thickner. In order to achieve the satisfactory filtration of the supernatant brine through the perfect separation of the solid matters, the brine feed rate has to be reduced and the filtering apparatus needs to be washed frequently for regeneration, which causes increases in the brine purifying cost.